A typical condenser microphone is composed of a voltage bias element, Vbias, (commonly an electret), a diaphragm/backplate pair forming a capacitor which varies with sound pressure, and a Field Effect Transistor (FET) circuit for buffering the output signal. Miniature microphones used in hearing aids and other applications are typically electret condenser microphones. These are built with highly precision stamped metal parts, organic diaphragm films such as mylar and polyester, and highly charged electret films to bias the microphones. These microphones have certain short-comings. Their size has been reduced to the limits of manufacturability. Lack of uniformity in the stamping and assembly processes results in a large variability in sensitivity. Furthermore, temperature and humidity effects on the organic diaphragm film and electret result in long term drift in microphone performance.
In attempts to overcome the difficulties associated with traditional miniature microphones, various workers have tried to make solid state microphones using semi-conductor techniques. Such microphones using inorganic thin films have the potential to overcome the problems associated with conventional miniature microphones. However, attempts to build such solid state microphones have not been successful in achieving the necessary sensitivity simultaneously with good manufacturability.
Conventional microphones have rectangular diaphragm/backplate pairs typically measuring several millimeters on a side with a spacing between the diaphragm and the backplate of several tens of microns. An electret bias of several hundred volts is required to bring the microphone sensitivity to the desired range. In designing a solid state microphone, for example one from silicon, it is desirable to reduce the bias voltage to the 5 to 10 volt range in order to eliminate environmental stability problems with the electret. This voltage can conveniently be developed directly from the power supply or with conventional charge pump circuitry. Reducing the bias voltage value requires a commensurate increase in the ratio of the change of capacitance (.DELTA.C) to the capacitance (C) to maintain equivalent sensitivity. One method of maintaining the sensitivity of the miniature microphone is to reduce the gap between the diaphragm and the backplate to about 1 to 2 microns. It is also necessary to keep the mechanical compliance of the diaphragm (i.e., deflection vs. sound pressure level) to a level at least comparable to that in conventional microphones.
In a diaphragm there are two kinds of forces which resist deflection in response to pressure. The first kind of force includes plate bending forces which are proportional to the thickness of the diaphragm. These forces can be reduced by using a very thin film diaphragm. The second kind of forces which resist deflection include membrane forces which are proportional to the tension applied to the diaphragm. In the case of a thin film diaphragm, tension is not generally put in intentionally but is a result of the fabrication technique and of mismatches in thermal expansion coefficients between the diaphragm and the particular means utilized to hold the diaphragm in place.
Previous workers who have prepared solid state microphones have recognized the problem of residual tension in the diaphragm. Hohm and Hess, J. Acoust. Soc. Am. 85, 476-480 (1989) used a flat silicon nitride diaphragm with large residual tension. To reduce the tension, they implanted nitrogen to relax the nitride film. However, this technique is sensitive to implant dosage and energy, and to the thermal annealing cycle. It is difficult to control uniformity of the original tension across such a diaphragm and such a process may not impart long term stability to the diaphragm.
Bergqvist and Rudolf, Transducers 91, Proceedings of the International Conference on Solid-State Sensors and Actuators (IEEE, New York, 1991) pp.266-269, reduced membrane forces in a different fashion. They established a low tension diaphragm by using lightly doped single crystal silicon. While this was successful in reducing membrane tension, a parasitic capacitance was formed which canceled the advantage of the low stress diaphragm.
The present invention is provided to solve these and other problems.